BBCode is a markup syntax that is used in the osu!forums and, to a larger extent, the vast majority of forums on the internet to enable rich text formatting.
It is made up of tags that surround the text to enrich and, sometimes, attributes.

In osu!web, BBCode is used in forum posts, signatures and userpages.

Notes

While the post editor provides a few basic formatting tools, it is possible to manually write the BBCode.

BBCode tags are case insensitive.

BBCode Button Behavior

Without any text highlighted, clicking the button will surround the tags around the text cursor placement in the post editor.

With text highlighted in the post editor, clicking the button will surround the tags around the highlighted text.

Combining tags

Tags can be combined for fancier/richer formatting.

The order/nesting of the tags must be respected when combining the tags.
Failure to adhere to the order/nesting may break the post formatting.

Spoiler

This can be useful if you are talking about a critical scene in a TV show, movie, or anime but do not want to spoil it for others.
The reader, however, can still read the text by highlighting over the text.

Dedicated button:

Notes:

If you use the colour tag along with this, the colour will be used with a black background.

Lucky

Add a direct link to a page using Google's I'm Feeling Lucky button using the provided text.

Dedicated button:

Notes:

Know that this will not give everyone the same results.

Some search results are hidden due to language/location.

Syntax:

[lucky]...[/lucky]

List

Formats the text into a list.

In BBCode, there are two parts for making the list: the wrapper and the bullets.
Where the bullets are placed inside the wrapper.

Dedicated buttons:

List:

List type:

List bullet:

Notes:

All bullets must be surrounded by the list/list type tags.

These are the vaild arguments for TYPE:

(empty) - bulleted

1 - numbered

a - lettered (lowercased)

A - lettered (upper-cased)

i - roman numeral (lowercased)

I - roman numeral (upper-cased)

By default, bulleted lists uses sqaure bullets.

Lists inside lists are known to be buggy.

Syntax:

Where TYPE is one of the listed types above.

If TYPE is not defined, bullets will be used instead.

[list=TYPE]
[*]...
[/list]

Image

Show an image from an online image source.

The images can be sourced from anywhere, as long as the image exists from the given url.

Do not link the image directly from a local filepath
Using C:\Users\Name\Pictures\image.jpg will not work!

Please upload the image to reputable image sharing sites like imgur or puush.
Once the image is uploaded successfully, copy and paste the direct link provided in-between the image tags.

Also note, some websites don't appreciate direct links to their images (otherwise known as hotlinks).
Image sharing sites, like the ones listed above, are most likely okay with direct links since they are image sharing sites.

Dedicated button:

Notes:

If you have many images or large images, it is recommended to put them inside a Box.

Syntax:

Where LINK is a direct link to an image.

[img]LINK[/img]

Youtube

Embeds a YouTube video in your post.

Dedicated button:

Notes:

If you have many images or large images, it is recommended to put them inside a Box.

Syntax:

Where YT-ID is the YouTube video ID (11 characters long), not the whole url!

The YouTube video ID is located after the ?v= url parameter.

[youtube]YT-ID[/youtube]

Heading (v1)

Adds a big fancy pink heading.
This is usually used to denote a new section in yor post.

Dedicated button: none

Notes:

You must type the syntax yourself.

Syntax:

[heading]...[/heading]

Heading (v2)

Adds a big fancy purple heading with a horizontal line.
This is usually used to denote a new section in yor post.